When the correct feeder size isn't available??

There is nothing wrong with feeding multiple prey items that are an appropriate combined weight for a proper meal. The benefit to feeding one prey item is cost as well as ease of feeding.

When feeding two, I wait until the first prey item is swallowed and the snake has does the yawn and re-align the jaw thing and then offer the second. It's never been a problem and they have never refused. That being said, when they are gone I move up prey size and always prefer to feed one prey item.

As far as ordering in bulk, it is definitely the way to go. When ordering for a sub-adult snake that I know will outgrow the current prey item, I weigh the prey when it comes in and bag in bags of 5 (helps keep them "fresh") based on weight. Then feed in increased weights as the snake grows. I will also save a few of the smallest ones for when it's time to double up. They end up all being used and I save plenty of money over buying from a local store and paying ridiculous prices. Honestly, I could throw 10 away and I would still save money ordering bulk.

I've found that when I can't get the correct prey size or with fussy eaters the trick is to offer something they will eat and then offer the second. I resorted to stuffing a f/t rats head in the mouth as mine finished his mouse since he was really underweight at one point. After a while things turned around and I am now more worried my ball is fat than underweight. Ether way giving them multiple prey items isn't an issue I've got one royal that I fed 4 mice in succession just to use them up, all of which totalled between 10-15% of his body weight when combined.